1. Field of the Invention
The present invention relates to a measuring probe, and relates in particular to a probe having the capability of measuring the deflection of a stylus assembly from a zero or rest position defined by a resilient planar support. Probes of this type may be used for scanning the surface of a workpiece to determine its shape.
2. Description of Related Art
Such a probe is known, for example, from UK Patent Specification No. 1,573,447. This specification describes a probe in which a stylus assembly, which has a work-contacting stylus attached thereto, is supported in the rest or zero position (i.e. the position in which no external load is applied to the stylus) by a planar spring. Probes of this type have few moving parts and therefore have the advantage of being relatively cheap to manufacture.
However, these probes suffer from a problem in that the force required to produce a given deflection of the stylus from the rest position in the direction of its longitudinal axis, (hereinafter referred to as the Z axis of the probe), and the force required to produce the same magnitude of deflection in the sense of tilting the stylus relative to the Z axis are interdependent. That is to say, that any modification to the probe which alters the spring rate in respect of one of these motions, will also affect the rate in respect of the other.
For example, in the above described probe design, it is usually the case that the spring rate in the Z direction is greater than the spring rate in the sense of tilting of the stylus.
This gives rise to a problem when the probe is used as an analogue probe for scanning a workpiece surface, where the stylus is in continuous contact therewith, in that, when the stylus tip meets a change in the inclination of the surface which increases the pressure on the stylus in the Z axis, the stylus may slide sideways on the surface due to the lesser force restraining tilting thereof instead of remaining in the desired plane.
In many scanning operations the machine control system works on the basis of calculating from the probe outputs of X, Y and Z deflections, the vector which is normal to the surface, and then driving the probe in a direction normal to the calculated vector. Thus, where the probe readings are giving a false indication of the surface inclination, (due to sideways sliding of the stylus) the control will drive the probe in the wrong direction and this can slow down the scanning process.